I have tubs directly on heat with no air gap.
With glass tanks with the little lip on the bottom it's a bit different, though the heat pad is affixed with adhesive to the bottom of the tank. I leave an air gap so the cords don't have the weight of the tank on them concentrated into this tiny point (the lip). Some tanks have the lip build differently so it already kind of has "legs" where the corners are longer, and if you have that, no need to do anything special to raise the tank up.
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